Uses of large choke

[kalee20]

There are two limitations to how much current it can handle. One is the limit due to it overheating, the other is the limit due to its inductance dropping due to core saturation. If well-designed, it is likely (but not definite) that the two will kick in at similar current levels.

It's easy to see what the overheating limit is. Pass DC current through, and see how hot it is after a couple of hours. At 18 ohms, a good starting point would be 500mA (giving 4.5W dissipation). Increase the current (waiting each time!) till it gets too hot to touch the windings comfortably. Then you know the thermal limit.

The saturation limit is a bit harder. Really, what you want is to use it as a filter choke, increase the DC load, and see how output ripple changes with load. That might be hard to set up. Another way, if you have a Variac, is just to connect to the Variac in series with an ammeter and plot AC current against AC volts. It's likely you will get to a couple of hundred volts before current starts increasing more than it should by proportion (1.9H has a reactance of about 600ohms at 50Hz). Then just use DC current as the RMS value of AC where this happens.

If you do want to use as a filter choke, or an output choke as Argus says, then the rating to apply is whichever of these currents is the lower.

[Argus25]

Maybe, since it is just a plain choke it could be used in a setup to demonstrate the principles of inductance as an energy storage device.

If it was switched across a 12V supply with a transistor (with a diode across the transistor's collector-emitter to protect it) it would store about 0.4 Joules after the current leveled out to 12/18 amps. Then if you put a capacitor across it, the voltage on the capacitor would peak to about V (after the transistor switches off), where the energy stored, 0.4J = CV^2/2 (solve the equation for V after you insert your capacitor value).

Then you would have a high voltage pulse source, you could use it to light gas discharge tubes or large neon tubes from the 12V supply etc and demonstrate inductor energy storage principles. If you used a mosfet, the drain source diode is already there and you could drive it with pulses from a simple 555 timer IC, could be fun.

[broadgage]

As regards the original purpose of the choke, might it have been from a battery charger or PSU that was used with audio equipment and therefore had to be smooth DC.
Private telephone exchange for example ?

[Silicon]

Are the laminations interleaved like a conventional mains transformer, or are the 'E' laminations separated from the 'I' laminations?

[qualityten]

Thanks for the interest in my question and the comments which are educational in themselves. Silicon, I will take a closer look at the laminations tonight.

[kalee20]

I'd be really surprised if they were interleaved! If they were, you'd have a rather indeterminate (but very high!) inductance... The only use for being as a 1:1 autotransformer.

[qualityten]

Just answering the question in post #24, the E laminations are separated from the I laminations - but held together by generous dollops of varnish!

[Ed_Dinning]

Possible a "swinging choke", but using conventional lams instead of specially shaped ones?

Ed

[Diabolical Artificer]

What is a swinging choke and how does it work and how does one calculate the cap values? I've found very little online.

BTW QT, have you got any further testing your choke?

Andy.

[G6Tanuki]

A "swinging choke" is one which is designed to partially saturate in use. Generally intended to be used in applications where the current-demand varies widely [class-B amps] it's a 'choke-input' filter [where the rectifier-side capacitor of the traditional Pi-filter is either omitted or of very-much-lower-than-usual value]

At low currents the choke's full inductance is in service; as the average current increases the choke approaches saturation and so the effective inductance drops, thereby allowing the rectifier to 'see' more of the downstream smoothing-capacitance and so the voltage is more stable with varying loads.

I only ever came across them used in stuff like big public-address amplifiers and powerful [four 813s!] AM transmitter-modulators along with Mercury-vapour rectifiers, where the reduction/elimination of the rectifier-side capacitor worked to limit the peak current the [very low forward-resistance] mercury-rects would otherwise have allowed.

See http://www.vias.org/crowhurstba/crow..._vol3_025.html for a good explanation.

[Diabolical Artificer]

Thanks, much appreciated.

A.

[trobbins]

The 18 ohm DCR, and the physical size, indicate it would likely sit happily at 5-10W dissipation - which indicates 0.3 to 0.7A DC current, so that would be a substantial valve amplifier B+ supply.

The insulation resistance to case, and the voltage withstand capability is a bit harder to assess, so a safe target would be a mono or stereo amp based on line-output type valves that need to keep under a few hundred volts B+, rather than much more expensive KT88 or EL34 type high power beasts that more likely want greater than 600V B+. The lower B+ voltage naturally needs more current, which should suit that choke better.

Starting an amp project because on one part on the shelf is a little quirky, given that the choke is likely one of the easier parts to find. Nowadays, that level of inductance and DC current carrying capability is easily found in a 20-40W fluorescent light ballast - but no where near as cute looking if placed on top of a chassis with valves!

If you are keen, you can measure the inductance drop off with DC current using a simple-to-arrange AC + DC signal, as per the linked technique:
https://www.dalmura.com.au/static/Ch...easurement.pdf

[Diabolical Artificer]

" 0.3 to 0.7A DC current " - about 2 x 50w but you need about 430v HT really for that power.

A.

[trobbins]

Originally Posted by Diabolical Artificer

" 0.3 to 0.7A DC current " - about 2 x 50w but you need about 430v HT really for that power.

I guess one could go huge class A power amplifier path, but that really is a purist target with money to spare, and quite a room heater for 50W stereo.